If the user tries to build a package, but it can't be found: check if systemd is disabled and the package is in extra-repos/systemd, display a hint about it.
$ pmbootstrap build systemd
[17:37:02] NOTE: The package 'systemd' exists in extra-repos/systemd, but systemd is currently disabled
[17:37:02] ERROR: Could not find package 'systemd' in pmaportsFix #2398 (closed)
Details are in the commit messages.